The present invention relates to a method of and apparatus for producing compacted chopped strands of high density. More particularly, the invention is concerned with a method of and apparatus for producing a substantially rod-shaped compacted chopped strands from chopped strands having generally flat shape obtained by cutting a strand of long fiber such as glass fiber strand or a roving of the same.
In recent years, glass-fiber reinforced thermoplastic materials are finding spreading use as the materials of parts which are required to have high mechanical strength.
Various methods have been known as the method of producing articles from such glass-fiber reinforced thermoplastic materials.
According to one of these known methods, a mixture of pellets of a thermoplastic resin and chopped strands of glass fibers pelletized by an extruder, and the pellets thus formed containing glass fibers are then formed into an article by an injection molding machine. According to another known method, the mixture of the chopped strands of glass fiber and resin pellets is directly supplied to an injection molding machine and formed into the article.
In these methods, the unity or integrality of the chopped strands is an important factor which largely affects various process characteristics and formability in the steps of preparing the mixture of the resin pellets and chopped strand, as well as in the final forming step, and also various physical properties of the formed article. Namely, any cracking, napping and separation of the chopped strands in these steps not only deteriorate the process characteristics and formability in such steps but also makes it difficult to achieve various strength characteristics and other physical properties such as homogeneity of the formed article.
On the other hand, there is a trend for labor saving, rationalization and systematization in the field of production of glass-fiber reinforced thermoplastic articles. This in turn requires a certain change in the form of chopped strands of glass fibers. Namely, hitherto, the fiber length of glass fibers for the production of glass-fiber reinforced thermoplastic generally ranged between 3 and 6 mm but, recently, there is an increasing demand for shorter chopped strands of, for example, 1 to 3 mm and having a higher integrality and density, in order to meet the operation characteristics and to make full use of abilities of machines such as automatic weighing machine, air conveyor and so forth which are used to comply with the request for the labor saving, rationalization and systematization. It is to be noted also that there is an increasing requirements for homogenization and smoothing of the glass-fiber reinforced thermoplastic articles themselves, as the use of such articles spreads to the field of small-sized parts. The demand for shorter and denser chopped strands is accelerated also by these requirements.
In this connection, the conventional chopped strands suffer a vital problem. Namely, according to the prior art, the chopped strands are produced to have flat form due to the characteristics of the forming process and the requirement by cutting method, and this flat form considerably deteriorates the process characteristics in the automatic weighing and air conveying, as well as formability. More specifically, in the ordinal glass fiber forming method, glass filaments are drawn from a bushing or orifice plate of a spinning furnace, gathered into strands and taken up on bobbins with large tension which is required to form filaments and to overcome the frictional force caused by contacting a sizing applicator, gathering shoes, traverse and so forth, so that the strands wound on the bobbins inevitably have flattened cross-section. The taken-up strands are dried into cakes. The strand of dried cake is cut into chopped strands. Immediately before the cutting, an aqueous coating agent or lubricant is applied to the cake. Such steps, however, do not substantially change the flat configuration of the strands. The same applies also to the case where undried cake is cut directly or the case where an aqueous sizing agent is secondarily applied to the undried cake before the cutting.
There has been known another chopped strands forming method, referred to as a direct cut type, in which strands from a spinning furnace are directly supplied to a cutter and cut into chopped strands. In this case, the fiber drawing tension is produced by the adhesion of the strand to a peripheral surface of a feed roller of the cutter, and the fiber is inevitably flattened on the feed roller. In consequence, the chopped strand naturally has a flat configuration. Such strands having flat cross-section have large surface area and are generally bulky as a whole. In consequence, the process characteristics in the automatic weighing and air conveying in the forming process, and also the process characteristics in the step of preparation of mixture with resin pellets, are deteriorated undesirably. Furthermore, the flattened cross-sectional shape of the strand inherently reduces the resistance against external mechanical force, to undesirably increase the tendency of napping, cracking and separation to further deteriorate the process characteristics and formability.
The chopped strand is formed by gathering a large number of filaments, e.g. 800 to 2,000 filaments, into a strand and then cutting the strand into chopped strands of a predetermined length. The shorter length of the chopped strands increases the cutting impact force exerted by the cutter in relation to the binding force. The resistance of the chopped strand against the mechanical external force is reduced also by this reason.